Nuclear speed and cycle length co-vary with local density during syncytial blastoderm formation in a cricket

The blastoderm is a broadly conserved stage of early animal development, wherein cells form a layer at the embryo’s periphery. The cellular behaviors underlying blastoderm formation are varied and poorly understood. In most insects, the pre-blastoderm embryo is a syncytium: nuclei divide and move throughout the shared cytoplasm, ultimately reaching the cortex. In Drosophila melanogaster, some early nuclear movements result from pulsed cytoplasmic flows that are coupled to synchronous divisions. Here, we show that the cricket Gryllus bimaculatus has a different solution to the problem of creating a blastoderm. We quantified nuclear dynamics during blastoderm formation in G. bimaculatus embryos, finding that: (1) cytoplasmic flows are unimportant for nuclear movement, and (2) division cycles, nuclear speeds, and the directions of nuclear movement are not synchronized, instead being heterogeneous in space and time. Moreover, nuclear divisions and movements co-vary with local nuclear density. We show that several previously proposed models for nuclear movements in D. melanogaster cannot explain the dynamics of G. bimaculatus nuclei. We introduce a geometric model based on asymmetric pulling forces on nuclei, which recapitulates the patterns of nuclear speeds and orientations of both unperturbed G. bimaculatus embryos, and of embryos physically manipulated to have atypical nuclear densities.

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Replication Randomization Blinding Data that were quantified and presented in the manuscripts are provided in the Source Data. In addition, a dataset of 3D+ T nucleus positions and tracking links that supports the findings of this study is available at the GitHub repository for this project.
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For within-embryo analyses, we included all detectable nuclei from the beginning of the recording through the point when a syncytial blastoderm had formed. This was sample size was chosen because we considered it to be the maximum possible number of intra-embryo observations for the phenomena of interest. For patterns of nucleus behavior in 3D+ T datasets, this meant that we had >10,000 nucleus-time points for each analysis. For 2D+ T datasets, we followed an analogous approach, analyzing all nucleus-timepoints from when there were at least 25 detectable nuclei, and concluding when a syncytial blastoderm had formed.
We established criteria for exclusion before any datasets were collected for this project. Embryos were excluded from analysis if there were aggregations of nuclei, or if there was a recording failure during microscopy, or if the embryo failed to hatch after imaging had concluded. Otherwise, all imaged embryos were included.
Each of the core empirical results of the paper were repeated at least three times. In each case the reported findings replicated.
Embryos that were used for experimental treatment were randomly drawn by hand from a dish of embryos that had passed a predetermined set of inclusion criteria: (1) no ruptures to the eggshell, and (2) at least one zygotic division had occurred.
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used two strains of Gryllus bimaculatus. The Act-H2B-EGFP strain was described by Nakamura and colleagues (Curr Biol, 2010). The Act-mtdT line was generated in strain of G. bimaculatus that was founded in 2006, as described by Donoughe and Extavour (Dev Policy information about dual use research of concern

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Biol 2016). Embryos were laid by adult females. We did not determine the sex of embryos that were used in the study.
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